The process which begins with recognition of antigen by antigen-sensitive cells and culminates in an immune response by their progeny can only be understood if the nature and function of the component parts (lymphocytes) of the response are appreciated. The generation of both humoral and cellular responses involves several functional subclasses of lymphocytes, including helper T cells. These are defined as cells that are required for the expression of another cell's function. The experiments in this proposal are designed to carefully investigate a number of parameters related to the interaction of helper T cells with other types of effector cells, in particular cytotoxic T cell precursors and antibody-forming (B) cell precursors. The overall objectives are to characterize the requirement, nature, and specificity of helper T cells which interact with the cytotoxic precursors, to determine the relationship between helper T cells which interact with cytotoxic precursors and with B cells specific for histocompatibility antigens, and to examine qualitatively and quantitatively the interaction of different types of helper T cells, including antigen-specific and antigen-non specific helper cells, with effector cell precursors that respond to a single type of antigen.